It's Mario Party. Whaddya want? My kids love it, and its great for, well, parties.
I enjoy playing Mario Party with my four and six year olds, it makes a video game experience that the whole family can enjoy. And this is no mindless cliche, either. There aren't a lot of games that four and six year olds can get their heads around that adults can actually enjoy with them, and this is one of them.
And then there's the party aspect--how many games can you sit your parents, your wife's girlfriends, your grandma down at, and play with them? Hey grandma, let's play Halo 2! Oh, yeah, that's just alien ichor, don't let it bother you. Yeah I know the force feedback hurts your arthritis. Just play through, stupid octogenarian.
All that being said, it's not without its flaws. The mic games are literally worthless and you should immediately turn them off in the settings and use the mic for a swizzle stick at your first opportunity.
Anyways the equation changes for me a bit because this is the first Mario Party I've picked up for the Cube. Last one I owned was on my N64. If you bought all the other versions there's probably not much here for you.
Anyways. It is what it is. Chances are that if you are the kind of person who's gonna enjoy it you already went out and bought it.